The present invention relates generally to free-space radio frequency (RF) communication systems and, more particularly, to isolation systems that separate the receive signal from interfering signals in a RF communications system. Interfering signals include signals received by the communications antenna causing co-channel interference and transmit signal leakage of the antenna's transmitter signal into the receiver. Many electronic systems, such as radar and wireless communication systems, operate over a wide range of microwave frequencies. For example, many radar and communication systems operate in what is known as a frequency-hopping or a frequency-agile manner over very wide microwave bandwidths. The bandwidth can vary up to an octave or more, and the frequencies can range from the S to Ku bands. The main objectives of an isolation system used in a full-duplex communication system is to provide low transmitter-loss and a high degree of isolation over a wide dynamic range of frequencies.
Non-reciprocal devices, such as circulators, are commonly used to provide isolation between a transmitter and receiver in a microwave antenna system. However, the degree of isolation is severely limited by the operating frequency range, and a circulator's isolation and transmission deteriorate as the input ports become unmatched. Direct leakage between the transmitter and receiver is usually the primary cause of interference, particularly in systems that do not employ circulators. U.S. Pat. No. 5,373,301 discloses a means for canceling direct leakage in an antenna system that has simple resistive three-port junctions which use a signal derived from a dummy circuit receiving a large portion of the transmit signal. However, this design requires at least half of the signal power produced by the transmitter to be used to cancel the interference.
Environmental effects, such as temperature changes and aging, cause an antenna's impedance to change, resulting in changes in the impedance-matching within the antenna circuit and unbalancing of any cancellation signal synthesized and applied to the receiver for canceling transmit interference. U.S. Pat. No. 4,970,519 shows a circuit that adjusts the amplitude and phase of a cancellation signal in order to optimize cancellation of transmit interference at a receiver. Signal phase-adjustment is performed by adjustment of delay lines in order to equalize the propagation paths of the cancellation signal and leakage signal. The signal level at the receiver is used as an error signal and fed back in a "control loop" for adjusting the amplitude and phase of the cancellation signal on the basis of minimizing the error signal. As a result, the receive signal corrupts the error signal and cannot be entirely removed by cancellation or correlation using the transmit signal. In addition, the cancellation must have frequency-dependent amplitude and phase characteristics that closely match those characteristics of the transmit signal in order to attain effective cancellation over a broad spectrum of transmit frequencies. Because interference occurs until the cancellation signal's parameters are optimized, continual adjustment of the cancellation signal will cause an interference signal whose magnitude depends on the response-rate of the signal-optimization process. Finally, intermodulation and distortion products are produced by the non-linear response of ferrite materials, which are commonly used as part of the antenna structure or circulator. Such interference is commonly removed by filtering, which has the undesirable consequence of limiting the effective bandwidth of operation of the antenna.
Some techniques for reducing co-channel interference include frequency-separation, time-division, orthogonal polarization, and spatial separation. Further reduction of interference requires some type of cancellation. U.S. Pat. No. 5,432,522 shows a canceller that reduces cross-polarization interference in two orthogonally polarized channels. U.S. Pat. No. 5,515,378 applies adaptive phased-array technology to wireless communications in order to provide spatial multiplexing and demultiplexing of communications channels. This prior-art adaptive processing in an antenna array is essentially a cancellation process. Each element of the array has an associated electrical signal that is adjusted by a complex-valued weight, then summed to provide an antenna beam pattern having nulls (canceled responses) in a predetermined direction. Problems with this technique include the inability to resolve co-located or closely-spaced radio sources and increased side-lobe structures relative to main-beam magnitude that results when the width of the main beam is narrowed. If wide-band or multiple frequencies are transmitted, this causes distortion of the main beam and variance in the location of the nulls.